2,9-Dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid supported the growth of mercaptoethanol requiring L1210 cells in primary culture. This specific copper chelating agent removes trace copper in tissue culture media, indicating that the mercaptoethanol requirement is associated with susceptibility to copper-mediated cytotoxicity. A survey of congeners of the 1,10-phenanthroline series indicated that only the 2,9-dimethyl derivative is cytotoxic in the presence of copper. The 1,10-phenanthroline ligand, its other dimethyl derivatives and 2,9-dimethyl-4-,7-diphenyl-1,10-phenanthroline are moderately cytotoxic in tissue culture, but added copper reduces their cytotoxicity. 2,9-Dimethyl-1,10-phenanthroline plus copper was minimally effective against the L1210 leukemia in vivo but gave a significant increase in life span in mice bearing the P388 leukemia.